1. Field of Invention
This invention relates to dual channel type signal detector circuits and more specifically to improvements therein for achieving more accurate and reliable signal detection. The invention is considered useful for various signal detection applications and particularly desirable for use in digital magnetic data read systems with reference to which it is explained herein.
2. Description of Prior Art
Magnetic recording of binary digital data is customarily performed by effecting polarization changes or transitions in a magnetic storage medium. One of the primary objectives in such data storage is to increase the data packing density, that is the number of information bits recorded per unit length of the storage medium. To achieve this objective, magnetic recording of the data is generally carried out by means of various encoding techniques that have been developed for the purpose of reducing the number of transitions per bit, or per group of bits, while simultaneously assuring that the maximum interval between transitions remains short enough so that a self-clocking capability can be maintained for recovery of the data when reading from the storage medium. Nevertheless, if the transitions become too closely spaced as a consequence of unduly high packing density, so-called pulse crowding occurs because of inherent features of the magnetic recording and reading process. Pulse crowding is manifested in the course of reading from the storage medium by interference between read pulses arising from adjacent transitions. This interference occurs because the read pulses overlap in time to some degree when the transitions are too closely spaced and it is aggravated by asymmetry and phase distortion of the read pulses resulting from differential phase shift of the constituent frequency components thereof which causes broadening of the individual pulses. It is well known in the magnetic data storage art, however, that the read pulses supplied from a magnetic read head typically contain such distortion and accordingly various tenchiques have been developed in the art to provide compensation therefor. Suitable phase compensation may be provided, for example, by the use of phase equalization means as disclosed in U.S. Pat. No. 3405403 issued Oct. 8, 1968 to G. V. Jacoby et al. In any event, even in the absence of such distortion and asymmetry or the provision of compensation therefor, if the read pulses overlap due to pulse crowding, interference will occur between adjacent pulses causing variable amplitude and shifting of the peaks of the read signal, so-called bit shift or peak shift, with resultant errors either as a consequence of failure to detect a transition indicative of a data bit or false interpretation of noise in the read signal as representative of a data bit.
The manner in which amplitude variation of the read pulses may affect signal detection capability will be discussed subsequently in greater detail with reference to the description of the peferred embodiments of the invention, but for the moment discussion will be confined to the matter of peak shift which is undesirable inasmuch as the peaks are representative of the data transitions recorded on the storage medium. For accurate signal detection the relative time occurrence of the peaks must be preserved in order to recover the data. It is therefore common practice in the magnetic data storage art to provide some sort of compensation or equalization which acts to narrow the width of the individual read pulses so that they do not appreciably overlap and thus do not cause intolerable peak shift or amplitude variation of the read signal. This equalization may be provided in the process of recording the data on the storage medium by acting on the recording signal, so-called write compensation, as exemplified by U.S. Pat. No. 3,503,059 issued Mar. 24, 1970 L. E. Ambrico. Alternatively, narrowing of the read pulses may be accomplished in the course of reading by acting directly on the read pulses, so-called read equalization, as disclosed in U.S. Pat. No. 3516066 issued June 2, 1970 to G. V. Jacoby. The present invention is concerned with read equalization. Hence, the remaining consideration of the prior art will be limited generally to that technique and discussed ultimately in relation to a particular state of the art signal detector circuit which is improved by means of the principles of the present invention.
As mentioned above, data recovery pursuant to reading from a magnetic storage medium is typically performed by sensing the occurrence of peaks of the read signal, and it is for that reason that provision is made for narrowing the read pulses so as to preclude interference therebetween which otherwise might intolerably shift the peaks. Additional factors attendant to pulse narrowing and which affect data recovery must also be considered, however, other than simply narrowing the pulses sufficiently to avoid interpulse interference. For instance, the more a pulse is narrowed, the greater its bandwidth becomes thus requiring a substantially commensurate increase of the read system bandwidth with an accompanying increase in noise. This is undesirable because noise which is at or near the peak of a read pulse can act to shift the peak of the pulse, so-called noise induced peak shift. It is therefore important to provide equalization which appropriately acts on the relatively broad read pulses so as to provide a degree of pulse narrowing sufficient to eliminate or at least substantially reduce interpulse interference, whereby peak amplitude variation and bit shift are satisfactorily avoided, but the pulse narrowing must not be so great as to substantially increase the required system bandwidth and thus unduly increase noise in the read system. To obtain this result, the individual read pulses should be narrowed so as to have a contained amplitude spectrum, that is, a spectrum of limited frequency range. The point is that since bit shift can be caused by both interpulse interference and noise, a tradeoff should be made with regard to pulse narrowing which has the affect of advantageously reducing interpulse interference but unfortunately concurrently enhancing noise. It should be noted though that noise enhancement due to pulse narrowing can be offset to some extent if the equalization is such that the narrowed pulses have an amplitude spectrum permitting reduced amplitude boost. This will be explained further hereinafter in connection with the detailed description of the invention. Increased noise can cause still other deleterious affects, however, which will be discussed in the following paragraph with specific reference to a dual channel detector circuit.
A dual channel signal detector circuit of the kind to which the invention relates is disclosed in U.S. Pat. No. 3631263 issued Dec. 28, 1971 I. H. Graham et al and comprises parallel connected peak detection and gate generation channels coupled to receive the read signal. The peak detection channel senses the peaks of the read signal in conventional manner to produce data pulses corresponding thereto, but as explained above, the occurrence of noise coincident with the read pulses can act to shift the pulses from their proper time positions thereby resulting in errors as a consequence of noise induced bit shift. The peak detection channel also responds to noise peaks in the baseline section of the read signal intermediate the individual read pulses and produces spurious non-data pulses representative of such noise along with the desired data pulses. These non-data pulses will not cause errors in a dual channel circuit where the gate generation channel responds to the read signal to produce successive gating pulses corresponding only to the individual read pulses. The pulses provided at the output of the peak detection and gate generation channels are then applied to an AND circuit for gating the data pulses to the exclusion of noise or non-data pulses. It will be appreciated, though, that noise present in the baseline regions of the read signal could generate erroneous gating signals which would act to gate non-data signals produced at the output of the peak detection channel and thus falsely result in interpretation of noise pulses as representative of data. For these reasons it is important to restrict the read system bandwidth in order to preclude or at least reduce both noise induced peak shift and the production of spurious non-data pulses in the peak detection channel and to avoid generation of erroneous gating signals in the gate generation channel.
From the foregoing comments it will be appreciated that the signal applied to the peak detection and gate generation channels should have certain characteristics regarding the amplitude, time duration, amplitude spectrum and phase spectrum of its individual pulses and further regarding the flatness of the baseline sections of the signal intermediate the pulses so that both channels and particularly the gate generation channel can respond to the signal in a manner to assure accurate and reliable detection. Presently available signal detector circuits, including the dual channel type circuit to which the invention relates, provide various ones or combinations of these characteristics but nevertheless are lacking in one respect or another and especially in their ability to provide reduced noise enhancement and protect against generation erroneous gating pulses.